Permutation City

Greg Egan

If one could simulate a human being completely, at the level of
fundamental physics, would the simulation be human in some sense?
It's hard to argue otherwise without resorting to some kind of
essentialism, but one obtains some disconcerting results if one follows
through on such a functionalist metaphysics. If all that "matters"
are fundamentally mathematical relationships, then there ceases to
be any important difference between the actual and the possible.
(Even if you aren't a mathematical Platonist, you can always find
some collection of particles of dust to fit any required pattern.
In Permutation City this is called the "logic of the dust" theory.)

In the 2050 world of Permutation City, nothing as unrealistic as
physical level simulation is possible, but there are "Copies" of people.
These are ad hoc simulations employing knowledge from all levels of
biology — at the synapse and neurotransmitter level in the brain,
but at much higher levels of abstraction for "less important" organs.
These Copies are run in an environment simulated along similar lines, some
of it in great detail, some of it just sketched in. Due to processing
limitations they run at best at one-seventeenth of the rate of ordinary
human beings and sometimes much more slowly, depending on the available
processor power.

There is also something called the Autoverse, an extremely complex
cellular automaton (a souped-up version of Conway's "Life") which
produces a world broadly similar to ours but with different physics,
chemistry, and biology. This is so computationally intensive that only
simple micro-organisms have been discovered — created? — and they don't
seem to evolve. As a result the Autoverse is considered more of a toy
than a serious research proposition; it is just a hobby for freelance
programmer Maria Deluca. Copies and the Autoverse are the technological
innovations crucial to the plot, but Egan also has lots of other ideas
about what the future might be like, socially as well as technologically.

Paul Durham is convinced by the "logic of the dust" theory mentioned
above, and plans to run, just for a few minutes, a complex cellular
automaton (Permutation City) started in a "Garden of Eden" configuration
— one which isn't reachable from any other, and which therefore must
have been the starting point of a simulation. This is set up to run
Copies of Paul, Maria and sixteen rich Copies seeking immortality, and
to expand in such a way as to provide them with effectively unlimited
computing power. (In order to provide something interesting to watch,
this automaton is also set up to simulate an entire Autoverse world,
which is why Maria is involved.)

The idea is that the simulation need only run for a few minutes, at a
slow-down of 250 to 1, and when stopped will continue ("running on the
dust"), because it will be more plausible to its inhabitants that it
have done so. I didn't understand the need for this elaborate set-up,
but I guess it makes for a better story than "well, all possible worlds
exist, and I'm going to tell you about one of them".

Part two is set entirely within the simulated Permutation City.
Eventually the logic of the Autoverse (which has evolved intelligent
beings who, I think rather implausibly, manage to think up a simpler
explanation for the Autoverse than its original one) starts to erode the
logic of the automaton simulating it, and things fall apart. I found
this part was less engaging — if still interesting — and the actual
ending unsatisfying. I guess it was never going to be easy ending
a novel that could, without inconsistency, have had absolutely any
ending (including ones where causality just goes completely haywire).

Permutation City is as good as Quarantine, and won't disappoint
Egan fans. While I really enjoy the way Egan plays with philosophical
ideas, I would really like to see him write a more "ordinary" science
fiction novel. This would give him a chance to explore the wealth of
other, less profoundly metaphysical ideas he has produced.